Identify the two processes the games are running under, either ./srcds_i686 or ./srcds_i486 depending on your setup, then take note of the two PIDs (numbers) on the second column of the same lines, in the case of the example, 10533 and 10536.

then simply type:
'kill -9 10533'
'kill -9 10536'

to terminate the processes.

Another option would be simply to resume screen, by typing 'screen -raAd' and ending the processes by hitting 'ctrl+c'